Active thermography is a non-destructive examination (NDE) technique in which energy such as ultrasound or electromagnetic energy is applied to a test object. Discontinuities in the test object, including structural features and flaws, generate heat under such stimulation. A resulting temperature distribution on a surface of the object is detected with an infrared camera. Information about defects and the inner structure of the object can be obtained by evaluating an infrared image of the surface or a time series of such images. Each image may be digitized into picture elements, or pixels, with each pixel representing a small unit area on the surface. These digitized images can then be used for digital displays and for computer analyses in which the temperature/time series is processed and analyzed by pixel and in patterns of pixels.
Energy may be applied to a test object by electromagnetic induction, ultrasound, flash radiation, laser, hot air, or microwave excitation, for example. In a pulsed version of active thermography, the excitation power is switched on for a time period of typically 1 millisecond to over 1 second, depending on the application. The resulting energy-induced temperature distribution on a surface is temporary, constantly changing, and must be captured by a camera at each time point to be recorded. Images captured by the camera may be used to guide subsequent repairs by transposing the recorded image of a defect onto the surface of the object to guide repairs, which is a time consuming procedure and creates an opportunity for distortions or misplacement of the markings.
Another known NDE technique utilizes a vulcanizing rubber material containing specialized magnetic particles and a hardener, such as sold by Dynamos, Inc. under the product name MagRubber. The uncured rubber material is poured into a hole or other area of limited access in a ferro-magnetic part to be inspected. A magnetic field is applied to the part to cause the particles in the rubber to migrate and concentrate near regions of the article containing discontinuities, since such discontinuities concentrate the magnetic lines of flux. The rubber then cures and solidifies to form a rubber replica of the part, freezing the particles in their magnetically induced positions. The rubber replica is removed from the part, and its underside that formerly contacted the part is inspected to reveal dark lines caused by the accumulation of the particles corresponding to the discontinuities in the part.